How Active Camouflage Will Make Small Drones Invisible

British company Plextek Consulting wants to make drones impossible for their targets to spot. And rather than having to choose between camouflage patterns optimized for rocky desert terrain or for dense vegetation, they have developed electronic panels to cover a vehicle with adaptive camouflage that could display different patterns at the flick of a switch.

The eight-by-ten-inch panels are standard e-ink commercial displays. Although they are monochrome, filters provide shades of brown and green. Plextek first tested the idea on military vehicles in a project for the U.K. Defense Science and Technology Laboratory, or DSTL, in 2014. For camouflaging a military vehicle, the main challenge was fitting the panels around armor or other bolted-on external equipment. The potential cost of hundreds e-ink of panels might also be significant, too.

The U.S. and Canadian militaries experimented with a similar idea back in World War II

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However, the Plextek team realized that the same technology would be ideal for small tactical drones. They are virtually silent, thanks to electric motors, but even with a wingspan of a few feet, they can be spotted. Adaptive camouflage would make them much less conspicuous. Plextek says its active camo is a good fit for small drones because the panels are lightweight and require no power. The small area to be covered means they should not add significantly to the cost of a drone.

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Plextek director Nicholas Hill says that for aircraft, camouflage is not a matter of color but of luminance. An aircraft is easily visible when it is much lighter or darker than the sky behind it. E-ink panels allow that to be tweaked easily and quickly. The prototype has to be adjusted manually, but Hill says that later versions could adapt automatically based on input from an upwards-facing camera that assesses the background behind the aircraft.

People have known about using light and dark to hide planes for longer than you might think. The U.S. and Canadian militaries experimented with a similar idea back in World War II, when it was known as counter-illumination. The idea takes a cue from nature: Many animals have a coloration pattern called counter-shading, meaning they are lighter underneath than on top, reducing visibility produced by shadows. For WWII, the idea was applied to aircraft, which were visible as a dark shadow against the sky, by using artificial lighting. U-boats typically cruised along the ocean's surface, ready to dive as soon as they saw an approaching aircraft. If the underside of the aircraft could be made light enough to match the sky, then the aircraft could get much closer before it was spotted, giving the submarine less chance to dive before it could be depth-charged.

The lights required considerable power in daylight, and it was difficult to fit them without affecting the plane's aerodynamics. So, rather than lighting the entire underside of the plane, "Yehudi lights," named for a running gag on the Bob Hope radio show, were fitted in the nose and leading edges to lighten the plane's silhouette as seen from the front. Pilots could adjust their intensity. Tested on a TBM-3D Avenger, these lights reportedly could reduce the range at which an aircraft was spotted from 12 miles down to two. But Yehudi Lights never matured enough to reach active service, and the technique was kept secret after the war.

A similar idea was tried in Vietnam in a project codenamed Compass Ghost. The big US F-4 Phantoms were at a disadvantage because they could be seen at much longer range than their opponents' small MiG-21s. Under Compass Ghost, a special grey paint scheme and a set of nine lamps reduced the range at which the F-4 could be seen by up to a third.

For aircraft, camouflage is not a matter of color but of luminance.

Compared to these camo technologies of decades past, the modern e-ink version of counter-illumination is lighter and more responsive in terms of the shades of sky it can match, as the wings can be made both lighter and darker.

Plextek carried out initial tests of its adaptive camouflage this April with static, pole-mounted drones. Hill says the next stage of development should lead to real in-flight testing. This will involve work with scientists at Southampton University to reduce weight and improve aerodynamics by integrating the e-ink panels into the structure of the wings. That is: Rather than being stuck, the panels would form the wing itself. Hill suggests it might even be possible to have the wings turn translucent.

You can't get real invisibility with this kind of tech. But you don't need to. A small drone need only be hard enough to see that it will not attract the attention of casual observers. People rarely much spend much time looking up into the sky, and with no noise to alert them, they have little chance of spotting a tiny intruder that's the same brightness as the sky.